1. Field of the Invention
This invention relates generally to impressed current anodes used in cathodic protection systems for underground metallic structures and relates particularly to anodes having structure which substantially reduces or prevents deterioration of the anodes caused by end effect.
2. Description of the Prior Art
In the past, it has been known that an underground metallic structure has been subjected to chemical or electro-chemical attack which causes rust and other corrosion since the metallic structures normally include both anodic and cathodic areas and that the rust or corrosion occurs in the anodic areas while the cathodic areas remain substantially free of corrosion. It has been recognized that the reason for this is that a galvanic current flows from the ground to the cathodic areas and that the anodic areas discharge an electric current into the ground. It is known that a higher electric current from an impressed current anode system which is located in the general area of the underground metallic structure flows to such structure and overcomes the current discharge of the anodic areas to cause the entire underground metallic structure to become cathodic so that corrosion does not occur.
Heretofore many efforts have been made to provide anodes and anode systems for the cathodic protection of metallic structures and these have included deep anode systems, shallow anode systems, and systems for use in sea water. Initially sacrificial anodes were provided which emitted a galvanic current and these sacrificial anodes slowly disintegrated so that the useful life of the anode was limited. Some efforts were made to extend the life of the sacrificial anodes by covering portions of the anode surface with a dielectric material. However, care was required to permit sufficient current to flow to prevent corrosion of the structure. Some examples of this type of structure are shown in the U.S. Patents to Douglas U.S. Pat. No. 2,855,358; Vixler U.S. Pat. No. 3,012,958; and Shutt U.S. Pat. No. 3,354,063.
In order to extend the effective life of a cathodic protection system and to insure that sufficient current was present at the metallic structure, anodes were provided which were electrically connected to an AC source of electrical energy by a rectifier or the like so that an impressed DC electrical current which would be controlled to certain values was applied to the anodes. The anodes were made of iron, high silicon cast iron, steel, copper, graphite, magnetite, and other materials. Normally, in groundbeds, the anodes were embedded in a carbonaceous backfill material such as calcined petroleum coke, metallurgical coke, graphite and the like. An impressed current was applied to the anodes at a current density sufficient to cause the underground metallic structure to become cathodic. However, these anodes slowly deteriorated so that it was necessary to replace them every few years. An example of this type of structure is Tatum U.S. Pat. No. 3,725,669.
In a further effort to extend the life of the anodes, titanium and niobium anodes have been provided which were partially or completely plated with a noble metal such as platinum, gold, silver, or the like. In the partially plated type of structure, when an impressed current was applied to the anodes, the non-coated portions of the titanium or niobium did not discharge current because the substrate materials had a natural threshold voltage which caused the anode material to polarize and form a non-conducting film along the exposed exterior surfaces, while the current discharge occurred from the plated surfaces into the carbonaceous backfill material. This type of anode has been expensive but has had a longer life.
Some examples of this type of structure are the U.S. Patents to Baum U.S. Pat. No. 1,477,499; Anderson U.S. Pat. No. 2,998,359; Krause U.S. Pat. No. 3,929,607; British Pat. No. 866,577, and the following publications: Platinum Metals Review, Vol. 2, No. 2, April 1958, pages 45-47; Platinum Metals Review, Vol 4, No. 1, January 1960, pages 15-17; Corrosion Technology, February 1960, page 50; Corrosion Technology, January 1962, pages 14-16; Corrosion Technology, February 1962, pages 38-40; Corrosion Prevention and Control, October 1962, pages 51, 52 and 54.
Generally, these prior art anodes and particularly the anodes used in groundbeds, have been long slender anodes having a length of from 9 inches (23 cm) to 8 feet (244 cm) and a diameter of 1 inch (2.54 cm) to 6 inches (15.24 cm) which included a length-to-diameter ratio in excess of one.
Many of these prior anodes have failed prematurely due to a phenomena known as end effect or pencilling. The obvious problem caused by end effect is the consumption of the anode material, ordinarily at one or both ends, resulting in a shorter system life. A less obvious problem is the loss of the electrical connection to the anode while the majority of the anode remains intact. This is due to the fact that most of the anodes available have the electrical connection at one end of the anode. Loss of the connection to one anode in a system results in the inability to discharge any current from the affected anode. Assuming a constant current demand, this means that the remaining anodes of the system must contend with a higher current density which compounds the end effect phenomena resulting in a domino effect.
An early attempt to deal with end effect in deep anode systems involved stacking the anodes close together. This technique involved the mutual interference of current discharged from contiguous anodes and slowed the rate of attack on most of the anodes in the groundbed; however, end effect on the outer anodes tended to be magnified. When the outermost anodes deteriorated, the rate of attack on the next outermost anodes was more severe so that a domino effect again took place.
A later attempt involved the addition of extra anode material round the connection at the end of the anode. This technique only delayed the inevitable result.
A more recent attempt to negate the results of end effect involved locating the electrical connection in the center of the anode. This technique did not solve the problem of end effect but it extended the life of the anode since the connection area was the last area of the anode to be consumed due to end effect.
Our earlier U.S. Pat. No. 4,175,021 was directed to structure for substantially preventing the end effect phenomena and included the optimum segmenting arrangement of an anode for uniform current discharge without particular attention to other engineering considerations. As pointed out in the prior patent, the closer the anode segmenting arrangement approaches a length-to-thickness ratio of one, the more uniform the current density becomes and it has been found that a completely uniform discharge occurs if the anode is generally spherical in shape. However, other engineering considerations must be weighed including resistance-to-earth of the anode as well as the cost in order to produce an anode which is acceptable to the industry.